Enhancements To The Spanning Tree Protocol - Alcatel-Lucent 7750 SR OS Service Manual

VPLS Features

Enhancements to the Spanning Tree Protocol

To interconnect 7750 SR routers (PE devices) across the backbone, service tunnels (SDPs) are
used. These service tunnels are shared among multiple VPLS instances. Alcatel-Lucent's
implementation of the Spanning Tree Protocol (STP) incorporates some enhancements to make the
operational characteristics of VPLS more effective. The implementation of STP on the router is
modified in order to guarantee that service tunnels will not be blocked in any circumstance without
imposing artificial restrictions on the placement of the root bridge within the network. The
modifications introduced are fully compliant with the 802.1D-2004 STP specification.
When running MSTP, spoke SDPs cannot be configured. Also, ensure that all bridges connected
by mesh SDPs are in the same region. If not, the mesh will be prevented from becoming active
(trap is generated).
In order to achieve this, all mesh SDPs are dynamically configured as either root ports or
designated ports. The PE devices participating in each VPLS mesh determine (using the root path
cost learned as part of the normal protocol exchange) which of the 7750 SR devices is closest to
the root of the network. This PE device is internally designated as the primary bridge for the VPLS
mesh. As a result of this, all network ports on the primary bridges are assigned the designated port
role and therefore remain in the forwarding state.
The second part of the solution is to ensure that the remaining PE devices participating in the STP
instance see the SDP ports as a lower cost path to the root rather than a path that is external to the
mesh. Internal to the PE nodes participating in the mesh, the SDPs are treated as zero cost paths
towards the primary bridge. As a consequence, the path through the mesh will be seen as lower
cost than any alternative and the PE node will designate the network port as the root port. This
approach ensures that network ports will always remain in forwarding state.
In combination, these two enhancements guarantee that network ports will never be blocked and
yet maintain interoperability with bridges external to the mesh which are running STP instances.
Page 338 7750 SR OS Services Guide